linux-kernel - Re: [PATCH 00/12] x86: Rewrite 64-bit syscall code

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Date:	Tue, 8 Dec 2015 05:42:54 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/12] x86: Rewrite 64-bit syscall code


* Andy Lutomirski <luto@...capital.net> wrote:

> On Mon, Dec 7, 2015 at 1:51 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> > This is kind of like the 32-bit and compat code, except that I preserved the 
> > fast path this time.  I was unable to measure any significant performance 
> > change on my laptop in the fast path.
> >
> > What do you all think?
> 
> For completeness, if I zap the fast path entirely (see attached), I lose 20 
> cycles (148 cycles vs 128 cycles) on Skylake.  Switching between movq and pushq 
> for stack setup makes no difference whatsoever, interestingly.  I haven't tried 
> to figure out exactly where those 20 cycles go.

So I asked for this before, and I'll do so again: could you please stick the cycle 
granular system call performance test into a 'perf bench' variant so that:

 1) More people can run it all on various pieces of hardware and help out quantify
    the patches.

 2) We can keep an eye on not regressing base system call performance in the
    future, with a good in-tree testcase.

Thanks!!

	Ingo
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